"MRI-negative PET-positive" temporal lobe epilepsy: invasive EEG findings, histopathology, and postoperative outcomes

The aim of this retrospective study was to analyze invasive EEG findings, histopathology, and postoperative outcomes in patients with MRI-negative, PET-positive temporal lobe epilepsy (TLE) (MRI–/PET + TLE) who had undergone epilepsy surgery. We identified 20 patients with MRI-/PET + TLE (8.4% of all patients with TLE who had undergone surgery; 11 men, 9 women). Of the 20 patients, 16 underwent invasive EEG. The temporal pole and hippocampus were involved in the seizure onset zone in 62.5% of the patients. We did not identify a lateral temporal or extratemporal seizure onset in any patient. Of the 20 patients, 17 had follow-up periods > 1 year (mean follow-up = 3.3 years). At the final follow-up, 70.6% patients were classified as Engel I, 5.8% of patients as Engel II, and 11.8% of patients as Engel III and IV (11.8%). Histopathological evaluation showed no structural pathology in any resected hippocampus in 58% of all evaluated temporal poles. The most common pathology of the temporal pole was focal cortical dysplasia type IA or IB. MRI–/PET + TLE should be delineated from other “nonlesional TLE.” The ictal onset in these patients was in each case in the temporal pole or hippocampus, rather than in the lateral temporal neocortex. Standard surgery produced a good postoperative outcome, comparable to that for patients with lesional TLE. Histopathological findings were limited: the most common pathology was focal cortical dysplasia type I.